Lithography is widely used in various industrial applications, including the manufacture of integrated circuits, flat panel displays, micro-electro-mechanical systems, micro-optical systems etc. Generally speaking, the lithography process is used for producing a patterned structure. During the manufacture of integrated circuits, a semiconductor wafer undergoes a sequence of lithography-etching steps to produce a plurality of spaced-apart stacks, each formed by a plurality of different layers having different optical properties. Each lithography procedure applied to the wafer results in the pattern on the uppermost layer formed by a plurality of spaced-apart photoresist regions.
To assure the performance of the manufactured products, the applications of the kind specified above require accurate control of the dimensions of the sub-micron features of the obtained pattern. When dealing with wafers, the most frequently used dimensions are the layer thickness and the so-called “critical dimension” (CD). CD is the smallest transverse dimension of the developed photoresist, usually the width of the finest lines and spaces between these lines. Since the topography of the measured features is rarely an ideal square, additional information found in the height profile, such as slopes, curves etc., may also be valuable in order to improve the control of the fabrication process.
Generally, an ordinary optical microscope can be used for measuring features' dimensions. A microscope is practically capable of measuring line width with a resolution of no less than 0.1 μm. The current high-performance semiconductor devices, however, have features' dimensions of 0.18 μm, and require CD measurement with the resolution of a few nanometers.
Several Optical CD (OCD) measurement techniques recently developed rely on imaging a certain test pattern which is placed in a special test area of the wafer. These techniques utilize various methods aimed at amplifying tiny differences in the line-width to obtain macroscopic effects that could be resolved by visible light, although the original differences are more than two orders of magnitude below the wavelength used. However, some of these techniques do not rely on fundamental physical effects, and thus could be more effective in some cases and less effective in others.
Another kind of technique utilizes scatterometric measurements, i.e., measurements of the characteristics of light scattered by the sample. To this end, a test pattern in the form of a grating is usually placed in the scribe line between the dies. The measurement includes the illumination of the grating with a beam of incident light and determining the diffraction efficiency of the grating under various conditions. The diffraction efficiency is a complicated function of the grating line profile and of the measurement conditions, such as the wavelength, the angle of incidence, the polarization and the diffraction order. Thus, it is possible to gather a wealth of data thereby allowing the extraction of information about the line profile.
Techniques that utilize the principles of scatterometry and are aimed at the characterization of three-dimensional grating structures and the determination of line profiles have been disclosed in numerous publications. Publications, in which diffraction efficiency was measured versus wavelength, include, for example the following:
(1) A. Roger and D. Maystre, J. Opt. Soc. Am, 70 (12), pp. 1483-1495 (1979) and A. Roger and D. Maystre, Optica Acta, 26 (4), pp. 447-460 (1979) describe and systematically analyze the problem of reconstruction of the line profile of a grating from its diffraction properties (the inverse scattering problem). A later article “Grating Profile Reconstruction by an Inverse Scattering Method”, A. Roger and M. Breidne, Optics Comm., 35 (3), pp. 299-302 (1980) discloses how the idea disclosed in the above articles can be experimentally used. The experimental results show that the line profile can be fitted such that the calculated diffraction efficiency will closely match the diffraction efficiency measured as a function of wavelength for “−1” diffraction order. The comparison of these experimental results with electron microscopy measurement showed a reasonable agreement.
(2) “Reconstruction of the Profile of Gold Wire Gratings: A comparison of Different Methods”, H. Lochbihler et. al., Optik, 98 (1), pp. 21-25 (1994) deals with the comparison of the results of several experimental techniques. Both optical transmittance and reflectance efficiencies were measured in the “0” order as a function of wavelength. By fitting the measurements to theoretical spectra calculated using diffraction theory, the grating profile was found. Comparison of these results with the results of X-ray diffraction efficiency and electron microscopy showed a good agreement.
(3) Voskovtsova, L. M. et al., Soviet Journal of Optical Technology 60 (9) pp. 617-19 (1993) studies the properties of gratings fabricated by replica technique. It has been found that the line profile of the hologram diffraction grating differs from the calculated sinusoidal profile. This difference leads to a difference in the spectral diffraction efficiency, an effect that was utilized for process control.
(4) Savitskii, G. M. and Golubenko, I. V, Optics and Spectroscopy 59 (2), pp. 251-4 (1985) describes a theory for the reflection properties of diffraction gratings with a groove profile which is a trapezoid with rounded corners. Such gratings can be fabricated by a holographic technique with photosensitive materials. It was found that the parameters of the trapezoidal profile, such as the depth of the groove, the width of a flat top and the slope of the side walls, affect the diffraction efficiency of the grating working in the auto collimation regime for the “−1” order.
(5) Spikhal'skii A. A., Opt Commun 57 (6) pp. 375-379 (1986) presents the analysis of the spectral characteristics of gratings etched into a dielectric material. It has been found that these characteristics can be significantly varied by slightly changing the grating groove profile.
(6) U.S. Pat. No. 5,867,276 discloses a technique for broadband scatterometry, consisting of the illumination of a sample with an incident light beam having a broad spectral composition and detecting a beam of light diffracted from the sample with a spectrometer. The technique is aimed at obtaining the spectrally-resolved diffraction characteristics of the sample for determining the parameters of the sample. The patent suffers from the following drawbacks: the measurements are done in the “0” diffraction order which is insensitive to asymmetries in the profile; and the analysis is done using the Neural Network (N.N.) method, which is sub-optimal by nature for applications requiring a high resolution. Additionally, the method does not take into account the need to focus the light onto a small spot, which is determined by the small area of the test structure allowed in the scribe line.
According to another group of publications, a monochromatic light source (e.g. laser) is utilized, and grating profile parameters are extracted from the measurement of the diffraction efficiency versus incidence angle. Such publications include, for example the following:
(A) S. S. H. Naqvi et al., J. Opt. Soc. Am. A, 11 (9), 2485-2493 (1994) discloses a technique that utilizes measurement of the diffraction efficiency in “0” order versus incidence angle to find the height of etched grating. Calculations are based on the Rigorous Coupled Wave Theory (RCWT), initially developed by Moharam and Gaylord and disclosed in M. G. Moharam and T. K. Gaylord, J. Opt. Soc. Am, 71, pp. 811-818 (1981), and several existing statistical techniques for the fitting stage.
(B) Raymond, J. R. et al., SPIE 3050, pp. 476-486 (1997) discloses a technique that utilizes a laser beam scanning with a range of angles to measure the diffraction efficiency versus incidence angle and to extract the line profile from the measured data.
(C) U.S. Pat. Nos. 4,710,642 and 5,164,790 disclose optical instruments which require to rotate the sample under test, which is definitely a disadvantage.
(D) U.S. Pat. Nos. 4,999,014; 5,889,593 and 5,703,692 disclose instruments employing angle-dependent intensity measurements without the requirement to rotate the sample. According to these techniques, different optical arrangements are used for providing the changes of the angle of incidence of an illuminating monochromatic beam onto the sample (wafer), without moving the sample. According to U.S. Pat. No. 5,703,692, the measurement is carried out by mechanically scanning the angle of incidence using a rotating block. The main disadvantages of such a technique are as follows: it requires the use of moving parts, the calibration of an angle in a dynamical situation, and has a limited angle range which does not provide enough information allowing accurate extraction of profile. According to U.S. Pat. No. 5,889,593, an optical arrangement includes a first lens that serves for focusing incident light onto a wafer at a range of angles, and a second lens that serves for focusing diffracted light onto a detector array. Although this technique does not need any moving parts, since the measurements are simultaneous, special care has to be taken to destroy coherence and avoid interference between the different light paths. Any suitable component for destroying the coherence always reduces the system resolution, thereby reducing the amount of obtained information.
In a third group of publications, the diffraction efficiency is measured when both wavelength and incidence angle are constant. In this case, information is extracted from the comparison of diffraction efficiency of several orders. This group of publications includes, for example, the following documents:
(I) U.S. Pat. No. 4,330,213 discloses a line-width measurement system using a diffraction grating. In this system, the intensities of first and second order light components are obtained to determine the line-width using empirical formulae.
(II) U.S. Pat. No. 5,361,137 discloses another example of the use of a conventional scatterrometry technique. Here, a set of intensities of the “1” or “2” diffraction order image of the set of “fixed-line width and variable-pitch-width” test gratings is recorded. From this set of intensities, line-width can be calculated.
Generally speaking, the conventional techniques use the following methodology in order to analyze the measured results:
First, a model is assumed for the grating profile having a number of parameters that uniquely define the profile. The user defines the required model (type of model) and sets the limits and the required resolution for each of the desired parameters.
Second, a spectral library is prepared using an optical model. The spectral library contains the calculated spectra for all possible profiles as defined by the user.
Third, given a measured spectrum, a fitting procedure finds the profile whose calculated spectrum included in the spectral library best matches the measured spectrum.